The present invention relates to a switched attenuator diode microwave power sensor and more particularly to a true average, wide dynamic range microwave power sensor utilizing a switched step attenuator on the sensor input.
U.S. Pat. 4,943,764 patent describes a way to make a power sensor so that each power sensing path is always nominally in it""s xe2x80x9csquare law regionxe2x80x9d when it is being monitored, and hence able to make accurate average power measurements on a radio frequency signal no matter what form of modulation is imposed on that signal. The first example is of a diode-attenuator-diode configuration that is claimed to give accurate average power measurements on any signal between xe2x88x9270 dBm and +20 dBm. The low power path shown in FIG. 6 which utilizes diodes D1 and D2 will indeed make accurate measurements between xe2x88x9270 dBm and xe2x88x9220 dBm. A problem comes when attempting to make measurements in the xe2x88x9220 dBm to +20 dBm range with the high power path. For all power levels of the RF signal, the low power path is exposed to the full strength of the signal, and at power levels above about xe2x88x9210 dBm, the low power path diodes will begin to change their RF impedance (video resistance) and also generate harmonics due to RF signal limiting. At power levels of +20 dBm, these high power effects from the low power path diodes will become severe, causing inaccurate power measurements due to RF signal limiting, harmonic generation, and increased input reflections from the changed input impedance. This same patent (U.S. Pat. No. 4,943,764) also describes a diode-thermocouple arrangement that does not need an attenuator, but instead uses an antiparallel pair of diodes for the low power path once again, while for the high power path a thermocouple power sensor is used. This diode-thermocouple arrangement will have the same problems of harmonic generation and RF impedance change of the low power path diodes degrading measurement accuracy as the diode-attenuator-diode arrangement did. While this degradation in measurement accuracy can be corrected for to a certain extent for CW signals, it will make the measurement of modulated signals with high peak to average ratios (such as CDMA signals) extremely inaccurate.
U.S. Pat. No. 5,204,613 entitled RF Power Sensor Having Improved Linearity Over Greater Dynamic Range describes a way to make a power sensor so that it utilizes a stack of 2 or more diodes in each arm of the anti-parallel pair in order to reduce the percentage change of the junction capacitance across the stack of diodes for a given RF input power compared to a sensor with a single diode in each arm of the anti-parallel pair. The explanation offered by this patent will only improve a sensor that has significant degradation in performance due to the change in it""s junction capacitance with power level. For more sophisticated diode structures, such as the Modified Barrier Integrated Diodes (MBID) used in some sensors, the junction capacitance is so low that it""s variation with power level causes minimal change in diode impedance relative to the 50 ohm load. Thus, MBID diode structures would gain no advantage from this technique, and yet they have extremely non xe2x80x9csquare lawxe2x80x9d response above xe2x88x9220 dBm, limiting their ability to make accurate average power level measurements on RF signals with high peak power to average power ratios at powers between xe2x88x9220 dBm and +20 dBm. Indeed, any of the commonly used diodes for RF power sensing applications would make inaccurate average power readings on RF signals with high peak to average ratios above xe2x88x9220 dBm even if there was an arbitrarily large stack of diodes that reduced the influence of capacitance variation across the diode junction on the measurement to 0, if indeed the variation in diode junction capacitance was all that contributed to diode non-linearities at high power.
Careful analysis of the causes of non xe2x80x9csquare lawxe2x80x9d operation for diodes used in RF power sensing applications has shown deviation from xe2x80x9csquare lawxe2x80x9d operation at higher power levels is found even in ideal diode models, and the cause of the deviation involves small signal approximations that are no longer valid at large signal levels. An analysis such as this is carried out in Application Note 64-1A, xe2x80x9cFundamentals of RF and Microwave Power Measurementsxe2x80x9d Hewlett-Packard Company, and is briefly summarized below:
i=Is(excex1vxe2x88x921)xe2x80x83xe2x80x83(Eqn. 1)
With i=diode current
Is=diode saturation current, constant at a given temperature
xcex1=q/nKT, (typically 40 voltsxe2x88x921)
v=voltage across the diode
Eqn. 1 can be written as a power series as:
i=Is(xcex1v+({fraction (1/2)}!)(xcex1v)2+({fraction (1/3)}!)(xcex1v)3=. . . )
It is the second, and other even-order terms in this series which provide rectification, and for small power level signals, only the second-order term is significant, so the diode is said to be operating in the xe2x80x9csquare law regionxe2x80x9d. When v is so high that the fourth and higher order terms become significant, the diode is no longer in the square law region, and is in the xe2x80x9ctransition regionxe2x80x9d One way to extend the xe2x80x9csquare law regionxe2x80x9d to higher power operation is to stack multiple diodes in series in order to cut down the RF voltage across each diode, extending the small signal range. As more diodes are added to the stack, the region of xe2x80x9csquare lawxe2x80x9d operation is extended by 20 log(n) in power while degrading your sensitivity by 10 log(n), where n is the number of diodes in the stack. Thus, the increase in dynamic range of the xe2x80x9csquare lawxe2x80x9d operating region, from the noise floor of the sensor to the start of the transition region, is 10 log(n).
The drawback of stacking multiple diodes in series is that present wide dynamic range sensors cover a 90 dB dynamic range from about xe2x88x9270 dBm to +20 dBm, although they can""t accurately measure modulated signals with high peak to average ratios above their xe2x80x9csquare law regionxe2x80x9d, which extends to approximately xe2x88x9220 dB. Any alternative solution should have a dynamic range as close to 90 dB as possible. In order to extend the xe2x80x9csquare law regionxe2x80x9d of operation of a power sensor to +20 dBm by using a diode stack, you may use the fact that the extension of xe2x80x9csquare law regionxe2x80x9d is proportional to 20 log(N) mentioned in the previous paragraph to find that you would need a stack of 100 diodes. Not only would a 100 diode long stack be physically large, but using the formula that the sensitivity of a diode sensor is degraded by 10 log(N), the new diode would have a noise floor of xe2x88x9250 dBm, and hence a dynamic range of only 70 dB.
Another technique used to try and make accurate wide dynamic range microwave power measurements for high peak to average ratio forms of modulation is to characterize a CW sensor for a given kind of modulation over the power range of interest, for example xe2x88x9270 dBm to +20 dBm. While this technique will work with relatively narrow bandwidth modulations, the necessity of running a high frequency signal up the cable connecting the power sensor to the power meter without degradation will limit this technique to lower frequency modulations. This will be a disadvantage as the modulation bandwidths of signals continue to increase.
It is an object of the present invention to provide a switched attenuator diode microwave power sensor and more particularly a true average, wide dynamic range microwave power sensor utilizing a switched step attenuator on the sensor input.
In one preferred embodiment, the present invention provides a switched attenuator diode microwave power sensor comprising means for receiving RF signals having wide dynamic power ranges; an anti parallel pair of sensor diode means for measuring the power level of the received RF signals; a switched attenuator means having a first low loss state for lower power range RF signals and a second attenuated state for higher power range RF signals; the switched attenuator means switching the received RF signals through the first low loss state to the sensor diode means when the received RF power level is at the lower power level, said switched attenuator means switching the RF signals through the second attenuated state to the sensor diode means when the received RF power level is above the lower power level range such that the sensor diode means operate in their square law region and measure the power level over the wide dynamic ranges of the received RF signals.